1. Technical Field of the Invention
The present invention relates to an image display apparatus, and particularly to an image display apparatus having a light emitting element in each of its pixels.
2. Prior Art
Among the image display apparatuses employing a light emitting element in each of its pixels, many reports have been made on EL displays using electroluminescent (hereinafter abbreviated as EL) elements.
In the active matrix type EL display, wiring lines for transmitting signals and currents are arranged in a matrix configuration, and a pixel circuit formed of thin film transistors (hereinafter abbreviated as TFTs), which are active elements, is incorporated in addition to the EL element within each of its pixels.
As methods for the pixel circuit to control light intensity of the EL element, there is a method by modulating a duration of time during which a pixel circuit supplies a current to an EL element, as reported in SID '00 DIGEST, PP. 924–927, FIGS. 1, 2 and 6.
FIG. 15 illustrates a conventional pixel using an EL element. A pixel 151 is composed of a pixel circuit and an EL element 156. The pixel circuit is composed of TFT 152–TFT 154 and a capacitor 155.
Connected to the pixel 151 are a signal line Dline for inputting a digital signal which is a display signal, a line Vline for supplying a current to the EL element 156, a signal line PS for supplying a signal for writing the display signal on the signal line Dline into the capacitor 155, and a signal line ES for supplying a signal for resetting the capacitor 155.
The pixel 151 can produce many gray scale levels of luminance by the following drive method.
In a case where luminance is generated which is represented by a 6-bit gray scale including 64 gray scale levels, for example, one frame period used for displaying one picture is divided into six sub-frame periods, and the following operation is performed during each of the six sub-frame periods.
At the beginning of one sub-frame period, a digital voltage signal bx, which is a display signal, is supplied to the signal line D1, and an H level pulse is supplied to the signal line PS, and thereby TFT 152 is turned ON, and the digital voltage signal bx is stored in the capacitor 155.
The capacitor 155 retains the digital voltage signal bx during the sub-frame period, and if the voltage bx is at the L level, since TFT 154 is ON, the EL element 156 is lighted, and if the voltage bx is at the H level, since TFT 154 is OFF, the EL element 156 is extinguished.
After a specified lighting time, the H level pulse is supplied to the signal line ES, TFT 153 is turned ON, thereby the capacitor 155 is reset, and TFT 154 is turned OFF. If the ratio between the specified lighting times of the six sub-frames are selected to be 32:16:8:4:2:1, and voltages corresponding to respective digital bits of the display data are supplied in the order beginning with the MSB (Most Significant Bit) as the digital voltage signals bx, average luminance of a pixel averaged over one frame period is proportional to the display data. Here, the H and L levels mean the binary voltages of the digital voltage signals.
The pixels 151 are arranged in two dimensions, and an image is displayed by writing display signals successively into the pixels.
The method of controlling the average luminance by varying the duration of the lighting time of the EL element in this way has an advantage that it is easy to produce multi-gray scale display good in linearity, because a current flowing through the EL element 156 does not depend upon display signals, and therefore the EL display can display an image whose brightness varies smoothly.